问题描述
例如，您可以对两个字段执行MongoDB涵盖的查询吗?
Can you perform a MongoDB covered query for two fields, for example
db.collection.find( { _id: 1, a: 2 } )
没有具有复合索引，例如
db.collection.ensureIndex( { _id: 1, a: 1 } )
，但是只为_id提供一个索引(默认情况下为该索引)，为字段"a"提供另一个索引，如
but instead having only one index for _id (you get that by default) and another index for field "a", as in
db.collection.ensureIndex( { a: 1 } )
换句话说，我想知道是否要对两个字段执行覆盖查询，我是否需要复合索引而只需要两个单(即非复合)索引，每个字段一个.
In other words, I'd like to know if in order to perform a covered query for two fields I need a compound index vs. needing only two single (i.e., not compound) indexes, one for each field.
推荐答案
查询仅使用一个 index .
您的示例将_id显示为索引的元素之一? _id在集合中需要唯一，因此使_id和其他内容的复合索引没有意义.
Your example shows _id as one of the elements of your index? _id Needs to be unique in a collection, so it wouldn't make sense to make a compound index of _id and something else.
如果您改为:
db.collection.ensureIndex( { a: 1, b: 1 })
然后，您可以根据需要独立地使用a索引，或与b一起使用复合索引.
You could then use the a index as needed, independently, or as a compound index with b.
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